How to Build the World's Longest Floating Bridge

The floating bridge on Seattle's Route 520 already has the distinction of being the longest one in the world. Now crews are preparing to overhaul the bridge and make it even longer. If you're wondering how they'll get a quarter-million tons of concrete to float, we found out.

How to Build the World’s Longest Floating Bridge

How to Build the World’s Longest Floating Bridge

Hulking engineering projects built with 230,000 tons of concrete generally don’t float. Washington state, however, knows a little about how to make so much mass actually buoyant.

Washington owns the four longest floating bridges in the world. The longest, State Route 520, connects Seattle to points east over Lake Washington, which dips to 214 feet deep. That means a straight-line suspension bridge is out of the equation, and a suspension bridge in water that deep would require a bridge tower the height of Seattle’s Space Needle. Conventional bridges have proven too expensive to build in deep waters with soft beds, so floating a bridge over Lake Washington pencils out as the only possible solution to carry the roughly 115,000 daily vehicles.

Now the Route 520 bridge is getting even longer. And as Washington State’s Department of Transportation (WSDOT) starts reconstructing the world’s longest floating bridge, originally built in 1963, it has spent years developing a new bridge design and even a specialized concrete and concrete-pouring method, setting a new standard for floating bridges worldwide. The new bridge, which should open by 2014, will extend the span from 7578 feet (about 1.5 miles) to 7710 feet, and feature 77 concrete pontoons serving as the foundation for a six-lane, 116-foot-wide bridge deck.

Here are the basics of a floating bridge: Crews build watertight concrete pontoons connected end-to-end, and then place the roadway on top (there are also supplementary stability pontoons off to the side). The weight of the water displaced by the pontoons is equal to the weight of the structure and vehicles, allowing the bridge to float. Once the pontoons are in the water, crews will drop 58 anchors, made of reinforced concrete and attached to the pontoons via steel cables nearly 3 inches thick. A typical anchor can weigh as much as 10 male African elephants; they will sink into the loose material on the bottom of the lake to hold the pontoons in place. At each end of the bridge, the anchors will be drilled directly into the ground. Altogether, this system keeps the roadway from swaying.

But the science behind a floating bridge doesn’t start with displacement. It starts with the concrete mix. A typical pontoon measures 360 feet long, 75 feet wide and 28 feet tall. It’s as heavy as 23 Boeing 747s, but has to float. Not only that, only about 6 feet of the pontoon will stay above water. The rest will be pushed below the surface, subject to the wear and tear of salt water. (Editor's note: The pontoons are being built in salt water, but their destination, Lake Washington, is fresh water.)